All of the choices already mentioned make perfect sense if you are running a conventional round beam and want the light to see where you're going. If the function of the light is to be seen by oncoming traffic then I would think that light position would be a lot less critical and a helmet would probably provide more flexibility in catching driver's attention.

Any shaped beam can be mounted lower and still be more effective than a round beam. Personally I believe in redundancy and currently run a shaped beam from the top of each fork leg in the city, and couple that with a helmet mounted spot beam. But the brakes on a road bike make that a little less practical than on something equipped with disks.

For off-road use I'd kinda agree that a bar-mount would give the best overall light if only restricted to one light, but a helmet mount has the benifit of lighting up whatever you're looking at and the combo works best for me. I like as little as possible on the bars.